The rat delta opioid receptor (OP1) will be characterized with matrix-assisted laser desorption time-of-flight mass spectrometry (MALDI-TOF MS). MALDI-TOF MS offers the high mass capability that is needed to analyze an intact receptor, the high detection sensitivity that is needed to detect a receptor, and the ability to analyze the mixture of unresolved peptides that is produced by trypsin digestion and that will be used for peptide mapping and for characterization of the OP (Other enzymes and cyanogen bromide will be used). It is important to study the intact, fully post- translationally modified receptor. Compared to the other methods that are used to study receptors, this MS-based method offers high sensitivity, rapid analysis, and optimum specificity. Several operating features of MALDI-TOF MS that are needed to study the OP will be optimized, including: the speed of sample analysis (MALDI-TOF can analyze 100 samples on a plate at a rate of ca. one minute per sample; that rate is important for peptide mapping and OP characterization); detection sensitivity will be improved to the fmol/amol level; molecular specificity will be optimized by using post-source decay (PSD) to obtain the amino acid sequence of tryptic peptides; and the mass accuracy of measuring the molecular weight (MW) of the OP1 will be optimized (0.1% level) by using delayed extraction (DE) and a reflectron (R).